Aleksy Patryn

Electronic and Optical Properties of Strontium Barium Niobate Single Crystals

Electronic structure of Sr x Ba1- x Nb2O6 crystals in the energy range involving the valence and conduction bands has been studied theoretically and experimentally using the DFT-based ab initio VASP code and the spectroscopic ellipsometry method with synchrotron radiation. The Sr0.61Ba0.39Nb2O6 sample of the congruently melting composition is characterized by the greatest difference of the experimental dielectric function ϵ(E) compared to the non congruently melting compositions of Sr x Ba1- x Nb2O6 (x = 0.40, 0.55, 0.65, 0.75). Electronic band structures of Sr0.60Ba0.40Nb2O6 and LiNbO3 crystals in the energy range of 6–8 eV and the corresponding dielectric functions ϵ(E) in the photon energy range of 3–10 eV have been found to be similar, what is caused by similar octahedral NbO6 complexes in the crystals.

Manifestation of phase transformations in optical spectra of Na0.5Bi0.5TiO3 crystals between 25°C and 350°C

The pseudo dielectric function ⟨ε⟩ (E) = ⟨ε 1⟩ (E) + i⟨ε 2⟩ (E) of sodium bismuth titanate Na0.5Bi0.5TiO3, has been obtained in the spectral range of electronic excitations (2–9.5 eV) by spectroscopic ellipsometry using the synchrotron radiation source BESSY II. The spectrum contains a broad absorption band at 4.2–4.5 eV. The temperature dependence in the range of 25–350°C is presented by the related susceptibilities ⟨χ 1⟩ (T) and ⟨χ 2⟩ (T) at 5 eV. A clear peculiarity in the temperature behavior of ⟨χ 1⟩ (T) and ⟨χ 2⟩ (T) between 180 and 320°C has been revealed and discussed. In this range an extremum-like temperature dependence of intensity of the light reflected from the sample surface, has been revealed and can be explained by the diffraction of light on the grid of the elastic domain structure of the sample.

Band structure and optical spectra of ferroelectric triglycine sulphate

First-principal calculations of the band structure, density of electronic states, and dielectric functions of the triglycine sulphate crystal (TGS) together with the corresponding experimental dielectric functions in the range of 4–10 eV have been presented. Calculations have been performed in the frame of density functional theory using the CASTEP code. The experimental dielectric functions ε′(ω) and ε″(ω) have been obtained for different temperatures by ellipsometry using the synchrotron radiation source BESSY II. The theoretical dielectric functions ε′(ω) and ε″(ω) of TGS crystal agree satisfactorily with corresponding experimental dependencies. The character and magnitude of temperature changes of dielectric functions in the vicinity of phase transition point depend on the crystal orientation and photon energy. The greatest maximum-like temperature changes of dielectric functions observed at the phase transition point are detected for the photon energy ω ≈ 9.5 eV.

Spectral Ellipsometry Study of SBN Single Crystals in Visible and Ultraviolet Region

The pseudo-dielectric function (E) = (E) + i(E) of strontium-barium niobate single crystals, Sr x Ba1-x Nb2O6 (SBN), was studied at room temperature for five nominal compounds indices x (x = 0.40, 0.55, 0.61, 0.65, 0.75) by spectroscopic ellipsometry using the synchrotron light ellipsometer in the photon energy range of 1.5– 10 eV. The spectra of (E) obtained appear to be similar to the analogous spectrum of LiNbO3 crystal, where the NbO6 octahedral groups are responsible for the features of (E). Differences in the dielectric function of SBN with various x have been found and analyzed. The nonlinear dependences of optical values on the compound index x have been found in SBN.

Electronic Properties of KDP and DKDP Crystals: Ab-Initio Calculations and Spectral Ellipsometry Experiment

Electronic band structure, density of states and complex dielectric function ϵ(E) of KDP (KH2PO4) crystal at Fdd2 and space groups of symmetry corresponding to the ferroelectric and paraelectric phases, have been calculated within the density functional theory using the VASP code. The experimental dielectric function ϵ(E) of KDP and DKDP (KD2PO4) crystals have been studied by the spectroscopic ellipsometry method in the photon energy range of 5–18 eV using synchrotron radiation. Temperature dependences of the dielectric function ϵ(T) and the intensity of reflected light IR (T) of KDP and DKDP crystals have been measured and discussed.

Dielectric properties of (NH4)2SO4 crystals in the range of electronic excitations

Spectra of the real and imaginary parts of the pseudo-dielectric permittivity, (1)(E) and (2)(E), of ferroelectric ammonium sulfate crystals, (NH(4))(2)SO(4), have been measured in the range of electronic excitations 4.0 to 9.5 eV by ellipsometry using synchrotron radiation. Temperature dependences of the corresponding susceptibilities, chi(1)(T) and chi(2)(T), obtained for the photon energy E = 8.5 eV, related to excitations of oxygen p-electrons, reveal sharp peak-like temperature changes near the Curie point T(C) = 223 K. The large temperature-dependent increase of the imaginary part of the susceptibility chi(2)(T), together with a simultaneous decrease of the real part of the susceptibility chi(1)(T), take place at the phase transition. These anomalies have been ascribed mainly to the SO(4) group of the crystal structure.

Structural phase transitions in ferroelectric crystals and thin films studied by VUV spectroscopic ellipsometry with synchrotron radiation

Results of thermo-optical investigations for lead germanate (Pb5Ge3O11), potassium dihydrogen phosphate (KH2PO4) ferroelectric crystals, and lead titanate (PbTiO3) thin films expitaxially deposited on (001) SrTiO3 substrate are presented in this article. The measurements were performed using VUV spectroscopic ellipsometry with synchrotron radiation in the temperature range comprising structural phase transitions (PTs) in the materials under study. Distinct anomalies in the temperature dependencies of complex dielectric permittivity and the intensity of reflected synchrotron light are found in the vicinity of ferroelectric-to-paraelectric PT point and discussed. Different temperature points of the characteristic anomalies at the PT of the real and imaginary components of the pseudo-dielectric function are observed for Pb5Ge3O11 and KH2PO4. It is crucial that anomalies in the measured temperature dependences for PbTiO3 thin films on SrTiO3 substrates are observed at the temperature near 105 K, corresponding to the PT in the SrTiO3 single crystal.

Optical spectra of triglycine sulfate crystals in the range of 7–33 eV and its changes at phase transition

Pseudo-dielectric functions ⟨ε1(E)⟩ and ⟨ε2(E)⟩ of triglycine sulfate (TGS) crystal have been obtained in the range of 7–33 eV for different temperatures close to ferroelectric phase transition by spectral ellipsometry using the synchrotron radiation source BESSY II. These dielectric functions ε1(E) and ε2(E) have also been calculated from first principles using CASTEP CAmbridge Serial Total Energy Package code together with the band structure and density of electronic states of the crystal. Experimental and theoretical dielectric functions agree satisfactorily. A characteristic decrease of temperature dependences of the dielectric losses tgδ(T) and related values just below the phase transition temperature T c have been found and discussed.

Effect of annealing on the Structure of thermal evaporated In2S3 thin films

Structural studies on In2S3 thin films deposited by vacuum thermal evaporation on glass substrates at a temperature of 240C, followed by annealing at 330°C and 400°C are presented. It was shown that the films were of amorphous in nature before annealing and after annealing the films became polycrystalline and showed βIn2S3 structure. The grain size increased and followed the usual crystal growth process as the annealing temperature increased. The annealing-induced changes of surface roughness were characterized by atomic force microscopy. Streszczenie. Przedstawiono wyniki badania struktury cienkich warstw In2S3 naniesionych metodą termicznego osadzenia na szkło w próżni w temperaturach od 240C i kolejnym wygrzewaniem w temperaturach 330C i 400C. Ustalono że warstwy miały amorficzną naturę przed wygrzewaniem, po wygrzewaniu wykazywały strukturę polikrystaliczną typu β-In2S3. Rozmiar ziaren powiększał się na skutek aktywnej nuklearyzacji i to przyczyniało się do zwykłej krystalizacji ze wzrostem temperatury. Zmiany zostały przeanalizowane za pomocą AFM. (Wpływ obróbki temperaturowej na termicznie naparowane cienkie warstwy In2S3). Słowa kluczowe: In2S3, cienkie warstwy, XRD widma, Raman widma, AFM obraz powierzchni

Determination of CdSxSe1-x thick films optical properties from reflection spectra

A method for determining the band gap value and the refractive index near the absorption edge from reflection spectra was tested for CdSxSe1-x films prepared using the screen-printing and sintering technique. Streszczenie: Przeanalizowano metodę wyznaczenia szerokości przerwy energetycznej i współczynnika załamania z pomiarów widma współczynnika odbicie warstw CdSxSe1-x otrzymanych metodami sitodruku i konsolidacji termicznej (sintering technique). (Wyznaczanie parametrów optycznych grubych warstw CdSxSe1-x z analizy widma odbicia).